Many systems are currently in use for monitoring data related to long-distance or toll calls for the purpose of billing the cost of the long-distance call to the calling subscriber. The data that is usually monitored includes the directory number of the calling subscriber, the directory number of the called subscriber, the time at which the call starts, and the duration of the call.
A typical central office includes a switching system which functions to interconnect the line of the calling subscriber either directly to the line of the called subscriber, or indirectly to the line of the called subscriber through a local trunk going to another central office if the call is a local call, or indirectly to the line of the called subscriber through a toll trunk going to a toll office if the call is a long-distance call. The switching systems commonly in use today can be classified either as electromechanical or electronic. By far the majority of switching systems are electromechanical and these include step-by-step and all-relay switching systems.
Considering now a central office including a typical step-by-step switching system, each of the subscriber lines connected to the central office is coupled through an associated line circuit to the stationary contact banks of a group of line finders. Movable contacts of each line finder are connected to movable contacts of an associated first selector which has a stationary bank of contacts to which are connected various trunks including one or more toll trunks.
Assuming now that one of the subscribers initiates the placing of a long-distance call, a resultant off-hook condition of the telephone instrument of the calling subscriber is detected by the corresponding line circuit which responsively causes central office battery to be applied to the calling subscriber line and actuates one of the line finders. The line finder then proceeds to step its movable contacts through its stationary contact banks until the calling subscriber line is found, at which time the calling subscriber line is cut-through to the associated first selector and dial tone is placed on the calling subscriber line. Thereafter, the calling subscriber dials the telephone number of the called subscriber. In response to a first digit of the dialed telephone number such as the commonly used "1" digit, the associated first selector steps its movable contacts through its stationary contact bank until an appropriate toll trunk is found, at which time the first selector cuts-through to the toll trunk. Accordingly, a unique circuit is completed from the telephone instrument of the calling subscriber to the toll office through the calling subscriber line, the corresponding line circuit, one of the line finders, the associated first selector, and the toll trunk. Subsequent digits of the dialed telephone number including the area code and the directory number of the called subscriber are then received by and stored in a register within the toll office.
Identification of the calling subscriber is usually provided by automatic number identification apparatus located at the central office. When the dialed telephone number has been stored in the register in the toll office, the toll office transmits a signal to the central office via the toll trunk which causes a line splitting circuit to split the toll trunk into a first portion going to the first selector and a second portion going to the toll office. The automatic number identification apparatus includes a register sender which is connected to the first and second portions of the toll trunk through the line splitting circuit and also includes an ANI matrix which is interconnected with the register sender and with each of the line circuits in the switching system. Upon actuation of the line splitting circuit, the register sender places a unique signal on the first portion of the toll trunk which appears only at the line circuit of the calling subscriber line due to the unique switching path provided through the first selector and the line finder. The ANI matrix detects that line circuit upon which the unique signal appears and forwards the corresponding directory number of the calling subscriber to the register sender. The register sender then transmits the directory number of the calling subscriber to the register in the toll office via the second portion of the toll trunk. When transmission has been completed, the line splitting circuit is deactuated by the register sender to restore the connection between the first selector and the toll office, and the long-distance call proceeds, with monitoring of additional call data (such as the duration of the call) being provided by the toll office. Further details concerning representative automatic number identification apparatus known to the prior art can be found in U.S. Pat. Nos. 3,522,385, Stepan, and 4,099,031, Proctor et al. (which is assigned to the assignee of the present invention).
In view of the increased costs of service, many telephone companies now intend to provide individual call billing for not only long-distance calls but also all local calls, with the cost of each such call being related to the distance between the calling and called subscribers and the duration of the call. It is therefore necessary to provide call data monitoring for all calls placed through the switching systems of the company. In the particular situation of electromechanical switching systems, the methods and apparatus currently used to provide call data monitoring for long-distance calls were not designed for and are not capable of providing call data monitoring for all calls. These methods and apparatus also do not provide for local monitoring of all data related to a call (rather, the complete call data is monitored only at the toll office). It is therefore necessary to devise new methods and apparatus which permit call data monitoring, including automatic number identification, for electromechanical telephone switching systems.